High-strength seamless steel pipe for high-pressure hydrogen vessel and manufacturing method therefor

Abstract

The purpose of the present invention is to provide: a high-strength seamless steel pipe which is for a high-pressure hydrogen vessel and has excellent hydrogen embrittlement resistance; and a manufacturing method therefor.　This high-strength seamless steel pipe for a high-pressure hydrogen vessel has a specific composition and has a structure in which the area ratio of tempered martensite is at least 95%, wherein at least 50 mass% of Mo contained in the steel is contained in precipitates, at least 50 mass% of Mo contained in the precipitates is contained in precipitates having a diameter of at most 50 nm, and the tensile strength TS is at least 850 MPa.

Description

High-strength seamless steel pipe for high-pressure hydrogen containers and its manufacturing method

[0001] The present invention relates to a high-strength seamless steel pipe for a high-pressure hydrogen container and a method for producing the same.

[0002] Fuel cell vehicles that use hydrogen as fuel emit little carbon dioxide (CO ２ ) and has excellent energy efficiency, ２ Fuel cell vehicles are expected to be a vehicle that can solve emissions and energy problems. In order to popularize fuel cell vehicles, containers with excellent strength and durability that can safely store high-pressure hydrogen of 35 MPa or more, especially 70 MPa or more, are needed for hydrogen stations that supply hydrogen to fuel cell vehicles and for on-board use to load hydrogen onto fuel cell vehicles, and development of such containers is underway.

[0003] For vehicle-mounted pressure accumulators that require weight reduction, a liner made of a lightweight material such as aluminum or resin and coated with carbon fiber reinforced plastic (CFRP) has been proposed. For example, Patent Document 1 describes a liner made of an Al-Mg-Si alloy that has excellent fatigue properties.

[0004] On the other hand, in the case of pressure accumulators used in hydrogen stations, there is no need to reduce the weight as much as in the case of in-vehicle pressure accumulators, and therefore pressure accumulators formed entirely from steel (e.g., Patent Document 2) and pressure accumulators in which a liner made of Cr-Mo steel is covered with carbon fiber or glass fiber (e.g., Patent Document 3) have been proposed.

[0005] On the other hand, low-alloy steels such as Cr-Mo steel are known to be embrittled by hydrogen, so materials for high-pressure hydrogen storage vessels of 35 MPa or more are limited to aluminum alloys and SUS316, which are less susceptible to deterioration by hydrogen.

[0006] However, because stainless steel such as SUS316 has low strength, if the hydrogen pressure is increased to, for example, 70 MPa, the storage container will need to be extremely thick, increasing its weight. This not only limits the size of the storage container, reducing the amount of hydrogen that can be stored in the container, but also increases the cost of the material, making it less economical.

[0007] Much research has been conducted to use low-alloy steels, which have lower material costs, instead of austenitic stainless steels as materials for high-pressure hydrogen storage containers. Patent Document 4 proposes a steel for high-pressure hydrogen environments that utilizes fine V-Mo-based precipitates as hydrogen trapping sites in the steel to make the hydrogen non-diffusible and suppress embrittlement due to diffusible hydrogen.

[0008] Patent Document 5 proposes a low-alloy high-strength steel that has excellent resistance to embrittlement in a high-pressure hydrogen environment, in which the tensile strength is controlled within an extremely narrow range of 900 to 950 MPa by subjecting Cr-Mo steel to a tempering treatment at a relatively high temperature in the thermal refining treatment.

[0009] As these prior art technologies show, in order to achieve cost reduction, the use of low-alloy steels with improved hydrogen embrittlement resistance as infrastructure materials for high-pressure hydrogen gas environments is being widely studied. Note that the high-pressure hydrogen gas environment referred to here is an environment with a total pressure of 1 MPa or more and a hydrogen partial pressure of 1 MPa or more.

[0010] JP 2009-024225 A JP 2010-037655 A JP 2009-293799 A JP 2009-074122 A JP 2009-046737 A

[0011] It is known that when low-alloy steel is used for hydrogen pressure vessels, etc., hydrogen embrittlement can reduce the tensile strength of the material and significantly accelerate the rate of fatigue crack propagation. High-strength steel can be used for pressure vessels, such as accumulators for high-pressure hydrogen, allowing for thinner wall designs. This has led to a growing need for high-strength steel to increase storage capacity and reduce product weight. However, when the steel strength exceeds 850 MPa, hydrogen embrittlement becomes pronounced, and fracture can occur at a value lower than the maximum tensile strength when tested in air.

[0012] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength seamless steel pipe for high-pressure hydrogen containers that has excellent resistance to hydrogen embrittlement, and a method for manufacturing the same.

[0013] The present inventors have conducted extensive research into various factors affecting strength and hydrogen embrittlement resistance, since achieving the above-mentioned objectives requires both high strength and hydrogen embrittlement resistance. As a result, they have found that for high-strength steel pipes with a tensile strength TS of 850 MPa or more, the presence or absence of intergranular fracture significantly affects hydrogen embrittlement resistance. Therefore, they have found that by dispersing and precipitating fine Mo precipitates in the steel structure, the Mo precipitates act as powerful hydrogen trapping sites, reducing hydrogen accumulation at grain boundaries and suppressing the occurrence of intergranular fracture. Furthermore, they have found that the greater the amount of Mo precipitates, the better the hydrogen embrittlement resistance.

[0014] Based on these findings, the inventors have come to the conclusion that in order to further improve the hydrogen embrittlement resistance of high-strength seamless steel pipes, it is necessary to increase the amount of Mo contained in precipitates, and in particular to adjust the amount of Mo contained in precipitates of 50 nm or less to an appropriate amount or more. [1] A steel sheet having a composition containing, in mass%, C: 0.20 to 0.50%, Si: 0.05 to 2.00%, Mn: 0.30 to 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.005 to 0.150%, N: 0.006% or less, Cr: more than 0.2% and 1.7% or less, Mo: more than 1.0% and 3.0% or less, Nb: 0.001 to 0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, and Ti: 0.003 to 0.025%, and wherein Mo / C, which is the ratio of the Mo content to the C content, is in the range of more than 2.0 to 12.0, with the balance being Fe and unavoidable impurities; A high-strength seamless steel pipe for high-pressure hydrogen containers, having a structure with an area ratio of tempered martensite of 95% or more, wherein 50% by mass or more of the Mo contained in the steel is contained in precipitates, and further, 50% by mass or more of the Mo contained in the precipitates is contained in precipitates with a diameter of 50 nm or less, and having a tensile strength TS of 850 MPa or more. [2] A high-strength seamless steel pipe for high-pressure hydrogen containers according to [1], which in addition to the above composition further contains, by mass%, one or more elements selected from: V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, and W: 3.0% or less. [3] A high-strength seamless steel pipe for high-pressure hydrogen containers according to [1] or [2], which in addition to the above composition further contains, by mass%, H: 0.0010% or less. [4] A high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of [1] to [3], further containing, in mass %, Ca: 0.0005 to 0.005% in addition to the above composition. [5] A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of [1] to [4], comprising: casting a steel pipe material having the above composition to form a slab; heating the slab at a temperature in the range of 1050 to 1350°C; hot rolling the slab to form a seamless steel pipe of a predetermined shape; after the hot rolling, cooling the seamless steel pipe at an average cooling rate equal to or greater than air cooling to a temperature at which the surface temperature is 200°C or less;３ A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to [5], wherein the pipe is reheated to a temperature in the range of from the transformation point to 1000°C or less, and quenched one or more times to a temperature where the surface temperature is 200°C or less, and after the quenching treatment, the pipe is tempered by heating to a tempering temperature of 600 to 740°C, and the average heating rate until the tempering temperature is reached is 0.5°C / min or more and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes. [6] A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to [5], wherein the casting speed in the casting is 1.8 m / min or less.

[0015] According to the present invention, a high-strength seamless steel pipe for high-pressure hydrogen containers having a high strength of tensile strength TS of 850 MPa or more and excellent hydrogen embrittlement resistance can be easily and inexpensively produced, which is of great industrial benefit. Furthermore, by using a production method that contains appropriate amounts of alloying elements and promotes the formation of Mo precipitates, it is possible to stably produce a high-strength seamless steel pipe for pressure containers that has both the desired high strength and excellent hydrogen embrittlement resistance.

[0016] [Component Composition] First, the reasons for limiting the composition of the high-strength seamless steel pipe for high-pressure hydrogen containers (hereinafter simply referred to as high-strength seamless steel pipe) of the present invention will be explained. Hereinafter, mass % in the composition will be simply expressed as %.

[0017] C: 0.20 to 0.50% Carbon (C) dissolves in the steel, contributing to an increase in its strength. It also improves the hardenability of the steel and contributes to the formation of a microstructure in which martensite is the main phase during quenching. To achieve these effects, the C content must be 0.20% or more. The C content is preferably 0.22% or more, more preferably 0.25% or more, and even more preferably 0.28% or more. On the other hand, if the C content exceeds 0.50%, cracks will occur during quenching, significantly reducing manufacturability. For this reason, the C content is set to 0.50% or less. The C content is preferably 0.45% or less, more preferably 0.40% or less. The C content is even more preferably 0.35% or less.

[0018] Si: 0.05 to 2.00% Si is added for deoxidation, but if the content is less than 0.05%, the deoxidation effect is insufficient. For this reason, the Si content is set to 0.05% or more. The Si content is preferably 0.10% or more. It is more preferably 0.20% or more, and even more preferably 0.30% or more. On the other hand, if the Si content exceeds 2.00%, the effect saturates, so the Si content is set to 2.00% or less. The Si content is preferably 1.00% or less, and more preferably 0.80% or less. Furthermore, if the Si content exceeds 0.50%, toughness and weldability deteriorate, so the Si content is even more preferably 0.50% or less.

[0019] Mn: 0.30 to 1.50% Like C, Mn is an element that improves the hardenability of steel and contributes to increasing the strength of steel. To achieve this effect, the Mn content is set to 0.30% or more. The Mn content is preferably 0.40% or more, more preferably 0.45% or more, and even more preferably 0.50% or more. On the other hand, Mn is an element that segregates in steel and locally hardens the steel. If a large amount of Mn is contained, it will form localized hardened regions, adversely affecting hydrogen embrittlement resistance. For this reason, in the present invention, the Mn content is set to 1.50% or less. The Mn content is preferably 1.20% or less, more preferably 1.00% or less, and even more preferably 0.80% or less.

[0020] P: 0.015% or less P is an element that not only segregates to grain boundaries in the steel structure to cause grain boundary embrittlement, but also segregates to locally harden the steel. In the present invention, P is an unavoidable impurity and is preferably reduced as much as possible, but up to 0.015% is acceptable. Therefore, the P content is set to 0.015% or less. The P content is preferably 0.008% or less. The P content is more preferably 0.005% or less, and even more preferably 0.003% or less. The lower the content, the better, but from the viewpoint of refining costs, the P content is preferably 0.0001% or more, and more preferably 0.001% or more.

[0021] S: 0.005% or less S is an unavoidable impurity, and most of it exists in steel as sulfide-based inclusions, which reduce ductility, toughness, and even SSC resistance. Therefore, it is preferable to reduce the S content as much as possible, but up to 0.005% is acceptable. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.003% or less, and more preferably 0.002% or less. The lower the content, the better, but from the viewpoint of refining costs, the S content is preferably 0.0002% or more. The S content is more preferably 0.001% or more.

[0022] Al: 0.005 to 0.150% Al is added as a deoxidizer, but if its content is less than 0.005%, it has no added effect. For this reason, the Al content is set to 0.005% or more. The Al content is preferably set to 0.010% or more, and more preferably set to 0.020% or more. On the other hand, if the Al content exceeds 0.150%, the cleanliness of the steel decreases and the toughness deteriorates, so the Al content is set to 0.150% or less. The Al content is preferably set to 0.130% or less, more preferably 0.100% or less, and most preferably 0.080% or less.

[0023] N: 0.006% or less N is present in steel as an unavoidable impurity, but it combines with Al to form AlN, and when Ti is contained, it forms TiN, which has the effect of refining crystal grains and improving toughness. For this reason, the N content is preferably 0.0005% or more, and more preferably 0.001% or more. However, if the N content exceeds 0.006%, the formed nitrides become coarse and significantly reduce toughness. For this reason, the N content is set to 0.006% or less. The N content is preferably 0.005% or less, more preferably 0.004% or less, and even more preferably 0.003% or less.

[0024] Cr: more than 0.2% and not more than 1.7% Cr is an element that increases the strength of steel by improving hardenability and also improves corrosion resistance. Cr also combines with C during tempering treatment to form M. ３ C, M ７ C ３ , M ２３C ６ (M is a metallic element) and improves temper softening resistance, and is an element that is particularly necessary for increasing the strength of steel pipes. ３ C-type precipitates have a strong effect of improving temper softening resistance. To obtain this effect, the Cr content is set to more than 0.2%. The Cr content is preferably 0.3% or more, and more preferably 0.5% or more. On the other hand, if the Cr content is set to more than 1.7%, a large amount of M ７ C ３ , M ２３ C ６ These form small Mo precipitates, which act as hydrogen trap sites and reduce hydrogen corrosion resistance. Furthermore, a large Cr content causes coarsening of Mo precipitates. Fine Mo precipitates coarsen due to aggregation and coalescence, which reduces the number density of the fine Mo precipitates and reduces hydrogen embrittlement resistance. For these reasons, the Cr content is set to 1.7% or less. The Cr content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.8% or less.

[0025] Mo: more than 1.0% and not more than 3.0% Mo is an element that forms precipitates and contributes to strengthening steel through precipitation strengthening, and effectively contributes to ensuring the desired high strength after reducing dislocation density by tempering. Mo also dissolves in steel and segregates at prior austenite grain boundaries, contributing to improving hydrogen embrittlement resistance. Furthermore, Mo has the effect of densifying corrosion products and suppressing the formation and growth of pits that serve as crack initiation sites. To achieve these effects, the Mo content is set to more than 1.0%. The Mo content is preferably more than 1.1%, more preferably more than 1.2%, even more preferably 1.3% or more, and most preferably 1.4% or more. On the other hand, when the Mo content exceeds 3.0%, acicular M ２ C precipitates and, in some cases, Laves phases (Fe ２ Mo promotes the formation of Cr(Al) and Mo(II) and reduces hydrogen embrittlement resistance. For this reason, the Mo content is set to 3.0% or less. The Mo content is preferably 2.8% or less, more preferably 2.5% or less, even more preferably 1.8% or less, and most preferably 1.5% or less.

[0026] Nb: 0.001 to 0.020% Nb forms precipitates or carbonitrides, contributing to increased strength of steel through precipitation strengthening and also contributing to the refinement of austenite grains. To achieve these effects, the Nb content is set to 0.001% or more. The Nb content is preferably 0.005% or more, more preferably 0.006% or more, and even more preferably 0.007% or more. On the other hand, coarse Nb precipitates tend to become crack initiation sites for hydrogen-induced cracking. Therefore, the presence of a large amount of Nb precipitates due to a Nb content exceeding 0.020% leads to a significant decrease in hydrogen embrittlement resistance in high-strength steel. Therefore, from the viewpoint of achieving both the desired high strength and excellent hydrogen embrittlement resistance, the Nb content is set to 0.020% or less in the present invention. The Nb content is preferably 0.015% or less, and more preferably less than 0.010%.

[0027] B: 0.0003 to 0.0030% B segregates at austenite grain boundaries and inhibits ferrite transformation from the grain boundaries, thereby improving the hardenability of steel even in trace amounts. To achieve this effect, the B content is set to 0.0003% or more. The B content is preferably 0.0007% or more, and more preferably 0.0010% or more. On the other hand, if the B content exceeds 0.0030%, it precipitates as carbonitrides, etc., reducing hardenability and therefore toughness. For this reason, the B content is set to 0.0030% or less. The B content is preferably 0.0025% or less. The B content is more preferably 0.0020% or less, and even more preferably 0.0015% or less.

[0028] O (oxygen): 0.0030% or less O (oxygen) is an unavoidable impurity present in steel as oxide-based inclusions. These inclusions act as initiating points in a hydrogen gas environment and reduce hydrogen embrittlement resistance. Therefore, in the present invention, it is preferable to reduce the O (oxygen) content as much as possible. However, excessive reduction leads to increased refining costs, so the O (oxygen) content is permissible up to 0.0030%. For this reason, the O (oxygen) content is limited to 0.0030% or less. The O content is preferably 0.0025% or less, more preferably 0.0020% or less. The O content is even more preferably 0.0015% or less. Although there is no particular lower limit, the O content is preferably 0.0010% or more.

[0029] Ti: 0.003 to 0.025% Ti combines with N during solidification of molten steel and precipitates as fine TiN. This pinning effect contributes to the refinement of austenite grains. To achieve this effect, a Ti content of 0.003% or more is required. If the Ti content is less than 0.003%, the effect is small. Therefore, the Ti content is set to 0.003% or more. The Ti content is preferably set to 0.005% or more, and more preferably set to 0.010% or more. On the other hand, if the Ti content exceeds 0.025%, the TiN becomes coarse, preventing the above-mentioned pinning effect from being exerted, and instead, toughness is reduced. Furthermore, the coarse TiN further reduces hydrogen embrittlement resistance. For these reasons, the Ti content is set to 0.025% or less. The Ti content is preferably set to 0.020% or less, and more preferably set to 0.015% or less.

[0030] Mo / C: More than 2.0 to 12.0 If the Mo / C ratio, which is the ratio of Mo content to C content, is less than 2.0, Mo is insufficient, resulting in a small amount of Mo precipitates formed, and therefore sufficient Mo precipitates are not formed to improve hydrogen embrittlement resistance. Therefore, Mo / C is set to be greater than 2.0. Mo / C is preferably set to be 2.5 or greater, more preferably 3.0 or greater, and even more preferably 3.5 or greater. On the other hand, if Mo / C is greater than 12.0, the Mo precipitates tend to become coarse, resulting in a decrease in toughness and hydrogen embrittlement resistance. Furthermore, since the coarsening of Mo precipitates also occurs due to the aggregation and coalescence of fine Mo precipitates, the number density of fine Mo precipitates is also reduced. For these reasons, Mo / C is set to be 12.0 or less. Mo / C is preferably 10.0 or less, more preferably 8.0 or less, and even more preferably 6.0 or less. Most preferably, it is 5.0 or less.

[0031] The above-mentioned components are the basic components, but in addition to the basic composition, optional elements may be contained, such as one or more selected from V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, and W: 3.0% or less, H: 0.0010% or less, or Ca: 0.0005 to 0.005%, or any combination thereof.

[0032] One or more selected from V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, and W: 3.0% or less. V, Cu, Ni, and W are all elements that contribute to increasing the strength of steel, and one or more of them can be selected and contained as necessary.

[0033] V: 0.30% or less V is an element that forms precipitates and carbonitrides and contributes to strengthening the steel. The V content may be 0% or more, but in order to obtain the above-mentioned effects, the V content is preferably 0.02% or more, and more preferably 0.03% or more. On the other hand, even if V is contained in an amount exceeding 0.30%, the effect saturates and no effect commensurate with the content can be expected, which is economically disadvantageous. For this reason, when V is contained, the V content is set to 0.30% or less. The V content is preferably 0.20% or less, and more preferably 0.15% or less.

[0034] Cu: 1.00% or less Cu is an element effective in improving toughness and increasing strength, but if the content is too high, weldability deteriorates. Therefore, when Cu is contained, the Cu content is limited to 1.00% or less. The Cu content is preferably 0.75% or less, more preferably 0.50% or less, and even more preferably 0.25% or less. The Cu content may be 0% or more, but to obtain the above effects, it is preferable to contain 0.01% or more.

[0035] Ni: 2.0% or less Ni is an element that contributes to increasing the strength of steel and also improves toughness and corrosion resistance. To achieve this effect, the Ni content is desirably 0.03% or more. The Ni content is more preferably 0.1% or more. On the other hand, even if Ni is contained in an amount exceeding 2.0%, the effect saturates, and no effect commensurate with the content can be expected, which is disadvantageous in terms of economy. For this reason, when Ni is contained, the Ni content is limited to 2.0% or less. The Ni content is preferably 1.5% or less, more preferably 1.0% or less, and even more preferably 0.5% or less.

[0036] W: 3.0% or less W is an element that forms precipitates and contributes to increasing the strength of steel through precipitation strengthening, and also dissolves and segregates at prior austenite grain boundaries, contributing to improving hydrogen embrittlement resistance. To achieve this effect, the W content is desirably 0.03% or more. The W content is preferably 0.1% or more. On the other hand, if the W content exceeds 3.0%, the effect saturates, and no effect commensurate with the content can be expected, which is disadvantageous from an economical standpoint. For this reason, when W is contained, the W content is limited to 3.0% or less. The W content is preferably 2.5% or less. The W content is more preferably 2.0% or less, even more preferably 1.5% or less, and most preferably 1.0% or less.

[0037] H: 0.0010% or less. H may be introduced into steel during various manufacturing processes. A high introduction amount increases the risk of cracking after solidification and degrades hydrogen embrittlement resistance, so it is important to reduce the amount of hydrogen in the steel. These effects are not a problem when the H content is 0.0010% or less. Therefore, when H is contained, the H content is set to 0.0010% or less. The H content is preferably 0.0008% or less, more preferably 0.0005% or less, and even more preferably 0.0001% or less. Since a H content of less than 0.00001% increases costs, the H content is preferably 0.00001% or more. The H content is more preferably 0.00005% or more. The hydrogen content is the amount of hydrogen remaining after forming into plates, steel pipes, etc.

[0038] Ca: 0.0005 to 0.005% Ca is an element that combines with S to form CaS, effectively controlling the morphology of sulfide-based inclusions. Through morphology control of sulfide-based inclusions, it contributes to improving toughness and hydrogen embrittlement resistance. To achieve this effect, when Ca is contained, the Ca content must be 0.0005% or more. Preferably, the Ca content is 0.001% or more. On the other hand, when the Ca content exceeds 0.005%, the effect saturates, and the effect commensurate with the content cannot be expected, which is disadvantageous from an economical standpoint. Therefore, when Ca is contained, the Ca content is limited to 0.005% or less. The Ca content is preferably 0.004% or less, more preferably 0.003% or less, and even more preferably 0.002% or less.

[0039] The balance other than the above components is composed of Fe and unavoidable impurities. As the unavoidable impurities, for example, Mg: 0.0008% or less and Co: 0.0008% or less are allowable.

[0040] [Steel structure] The high-strength seamless steel pipe of the present invention has the above-mentioned composition and further has a structure in which tempered martensite is the main phase, in which precipitates are present, and further in which precipitates having a diameter of 50 nm or less are present.

[0041] Main Phase: Tempered Martensite Phase In the high-strength seamless steel pipe of the present invention, a martensite phase is the main phase in order to ensure a high strength of 850 MPa or more in tensile strength (TS). However, in order to maintain the ductility and toughness required for a structure, the tempered martensite phase, which is obtained by tempering the martensite phase, is used as the main phase. Here, the term "main phase" refers to a single phase in which the tempered martensite phase accounts for 100% of the area, or a tempered martensite phase of 95% or more, including a secondary phase of 5% or less in area, which does not affect the properties. The tempered martensite phase is preferably 97% or more, and more preferably 98% or more. As described above, the tempered martensite phase may be 100%. The secondary phase in the present invention may be, for example, a bainite phase, a retained austenite phase, pearlite, or a mixture thereof.

[0042] The above-mentioned structure of the high-strength seamless steel pipe for high-pressure hydrogen containers of the present invention can be adjusted by appropriately selecting the heating temperature during quenching and the cooling rate during cooling according to the steel composition.

[0043] If the grain size number of the prior austenite grains is less than 8.5, the substructure of the martensite phase that is generated becomes coarse, resulting in a decrease in toughness. Therefore, it is preferable that the grain size number of the prior austenite grains is 8.5 or more. The grain size number of the prior austenite grains is preferably 9.0 or more, more preferably 9.6 or more, and even more preferably 10.0 or more. The grain size number of the prior austenite grains is most preferably 12.0 or more. There is no particular upper limit, and a smaller number is better, but for reasons such as difficulty in observation and appropriate evaluation, a value of 18.0 or less is preferred. The grain size number is a value measured in accordance with the provisions of JIS G 0551.

[0044] In the present invention, the grain size number of the prior austenite grains can be adjusted by changing the heating rate, heating temperature, and holding temperature during quenching, as well as the number of times quenching is performed.

[0045] Furthermore, in the high-strength seamless steel pipe for high-pressure hydrogen containers of the present invention, the concentration of Mo precipitates is adjusted within an appropriate range depending on their size in order to improve hydrogen embrittlement resistance. The Mo precipitates were identified by the extraction method described in Patent Document 6 and Reference Document 1 using filter filtration. To measure the Mo concentration of the precipitates, 10 mm square samples taken from a cross section perpendicular to the rolling direction of the steel pipe (a cross section perpendicular to the pipe axis: C-section) were electrolyzed in an electrolytic solution, and the precipitates attached to the steel billet surface were placed in a dispersible liquid and irradiated with ultrasound to extract the precipitates into an aqueous solution. The aqueous solution from which the precipitates were extracted was filtered, and the precipitates were separated by size. The precipitates classified by size were dissolved in respective solutions, and the Mo concentration was analyzed by ICP to calculate the Mo content of the precipitates of each size. In concentration analysis using ICP, the solution is introduced into plasma to emit an element-specific spectrum, and the concentration of the element in the solution can be determined from the light emission intensity, allowing the Mo concentration (mass %) in the precipitates to be calculated. This technique allows the Mo content in the entire precipitate to be calculated, and the proportion (mass %) of Mo contained in the precipitates to the total Mo contained in the steel can be determined from this value and the Mo content in the steel. Furthermore, the concentration of Mo in the solid solution in the steel was obtained by analyzing the concentration of the solution after electrolysis using ICP in accordance with Patent Document 7. Furthermore, the Mo content in the precipitates remaining on the filter was analyzed using ICP, and the Mo content in precipitates larger than 50 nm was analyzed. The proportion (mass %) of Mo contained in precipitates with a diameter of 50 nm or less to the total Mo contained in the precipitates can be determined by subtracting the Mo content in precipitates larger than 50 nm from the Mo content in all precipitates. [Patent Document 6] JP ​​2010-127791 A [Patent Document 7] JP 2009-031269 A [Reference Document 1] Ishida et al., Analysis of the Formation State of Fine Precipitates in Steel, Iron and Steel, Vol. 107, No. 08

[0046] Of the Mo contained in the steel, 50% or more by mass is contained in precipitates. The presence of Mo in the steel composition as precipitates improves its properties in a hydrogen environment. Increasing the amount of Mo contained in the steel does not have the same effect if it is dissolved. On the other hand, the greater the amount of Mo precipitates, the better the hydrogen trapping ability, and the greater the improvement is achieved when 50% or more of the Mo contained in the steel is contained in precipitates. Therefore, it is necessary for 50% or more by mass of the Mo contained in the steel to be contained in precipitates. It is preferable that 60% or more by mass of the Mo contained in the steel be present in precipitates. More preferably, it is 65% or more by mass, and even more preferably, it is 70% or more by mass. While there is no particular upper limit, it is preferable that 95% or less by mass of the Mo contained in the steel be present in precipitates. More preferably, it is 90% or less by mass.

[0047] Of the Mo contained in the precipitates, 50% or more by mass is contained in precipitates with a diameter of 50 nm or less. Mo precipitates trap hydrogen in steel, inhibiting hydrogen accumulation at grain boundaries and improving grain boundary strength in a hydrogen environment. However, when the size of the precipitates exceeds 50 nm, the hydrogen trapping ability decreases, reducing the effect on improving grain boundary strength. Therefore, it is necessary for a large amount of Mo to be contained in precipitates with a diameter of 50 nm or less. Here, the hydrogen trapping ability improves as the amount of Mo in the precipitates increases, and this is significantly improved when the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 50% or more by mass of the Mo in the precipitates. For this reason, the present invention is limited to cases where the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 50% or more by mass of the Mo contained in the precipitates. Note that it is preferable that the Mo contained in fine precipitates with a diameter of 50 nm or less accounts for 60% or more by mass of the Mo contained in the precipitates. It is more preferable that it is 65% or more by mass, and even more preferable that it is 70% or more. Although there is no particular upper limit, the Mo contained in fine precipitates with a diameter of 50 nm or less is preferably 95% or less by mass, and more preferably 90% or less by mass, of the Mo contained in the precipitates. Since smaller Mo precipitates have better hydrogen trapping ability, it is more preferable that the precipitate diameter is 20 nm or less. As mentioned above, coarsening of Mo precipitates occurs due to aggregation and coalescence of fine Mo precipitates, and therefore coarsening leads to a decrease in the number of fine Mo precipitates. Although there is no particular lower limit for the diameter of the target precipitates, it is preferable that it is 1 nm or more.

[0048] In the present invention, in addition to the precipitation of Mo precipitates, reducing nitride-based inclusions and oxide-based inclusions, which can serve as fracture initiation sites, is also important for improving hydrogen embrittlement resistance. Management of the molten steel refining process is particularly important. Hot metal pretreatment involves desulfurization and dephosphorization, followed by decarburization and dephosphorization in a converter. After that, the hot metal undergoes a hot-stirring refining process (LF) and a hot-stirring vacuum degassing process in the ladle. Sufficient processing time is then ensured for the hot-stirring refining process (LF), as well as the hot-stirring vacuum degassing process, and the RH reflux rate is controlled. Furthermore, when producing slabs (steel pipe material) using a continuous casting method, in order to reduce nitride-based inclusions and oxide-based inclusions, an inert gas seal is applied when the slab is poured from the ladle into the tundish, and electromagnetic stirring is applied in the mold to float and separate the inclusions. While the refining process is not limited to the above, management of the refining process is also important in such cases.

[0049] [Manufacturing Method] Next, a manufacturing method of the high-strength seamless steel pipe for high-pressure hydrogen containers according to the present invention will be described.

[0050] In the present invention, a steel pipe material having the above-described composition is heated and hot-rolled to form a seamless steel pipe of a predetermined shape. The seamless steel pipe for high-pressure hydrogen containers is preferably applied to hydrogen containers with a hydrogen pressure of 1 MPa or more, and more preferably 20 MPa or more. The upper limit of the hydrogen pressure is not particularly limited, but is intended to be up to 120 MPa.

[0051] The steel pipe material (hereinafter also simply referred to as steel material) used in the present invention is preferably produced by melting molten steel having the above-described composition using a conventional melting method such as a converter, and then forming a slab (round slab) using a conventional casting method such as continuous casting. The slab may be further hot-rolled to form a round slab of a predetermined shape, or may be formed into a round slab by ingot-making and blooming.

[0052] In the following explanation, the manufacturing method will be explained using an example in which the steel pipe is a seamless steel pipe, but it goes without saying that electric resistance welded pipes and UOE steel pipes can be manufactured by performing treatment so as to have a similar thermal history. ３Similar properties can be obtained by manufacturing an electric resistance welded pipe by hot rolling a steel plate at a temperature in the range of from the transformation point to 1000°C, subsequently subjecting the steel plate to a quenching treatment at least once in which the steel plate is rapidly cooled to a surface temperature of 200°C or less, and then subjecting the steel plate to a tempering treatment in which the steel plate is heated to a temperature in the range of 600 to 740°C after the quenching treatment, and then welding the steel plate after the tempering treatment in which the average heating rate until the tempering treatment temperature is reached is 0.5°C / min or more and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

[0053] The steel pipe of the present invention can be produced by sequentially carrying out the following steps (1) to (3): (1) a step of adjusting the composition of a steel pipe material and then casting it, (2) a rolling step of heating and rolling a slab (cast material) to obtain a steel pipe, and (3) a step of cooling and tempering the steel pipe obtained in the rolling step. Each step will be explained below. Note that, unless otherwise specified, temperatures in the following explanations refer to the temperatures at the surface of the slab or steel pipe.

[0054] [Casting process] Casting speed: 1.8 m / min or less If the casting speed is too fast, the number of inclusions increases and hydrogen embrittlement resistance deteriorates, so the casting speed is preferably 1.8 m / min or less. The slower the casting speed, the more the hydrogen concentration and inclusions in the steel can be reduced, and this effect is more pronounced at 1.0 m / min or less, so the casting speed is more preferably 1.0 m / min or less. It is even more preferably 0.5 m / min or less, and most preferably 0.1 m / min or less. There is no particular lower limit, but it is preferably 0.01 m / min or more because of the difficulty of controlling the equipment.

[0055] [Heating Step] The slab having the above-described composition is heated for hot rolling. The slab is not particularly limited, but for example, a billet obtained by a conventional continuous casting method can be used.

[0056] Heating temperature: 1050 to 1350°C. If the heating temperature is less than 1050°C, the precipitates in the steel pipe material will not dissolve sufficiently. Therefore, the heating temperature is set to 1050°C or higher. The heating temperature is preferably 1100°C or higher, and more preferably 1150°C or higher. On the other hand, if the heating temperature exceeds 1350°C, the crystal grains will coarsen, and precipitates such as TiN precipitated during solidification will also coarsen, and cementite will also coarsen, resulting in a decrease in steel pipe toughness. Furthermore, if the heating temperature exceeds 1350°C, a thick scale layer will form on the surface of the steel pipe material, which will cause surface defects during rolling and increase energy loss, which is undesirable from the perspective of energy conservation. For these reasons, the heating temperature is limited to 1350°C or lower. The heating temperature is preferably 1300°C or lower. The heating temperature is more preferably 1250°C or lower.

[0057] [Hot Rolling Process] Next, the slab heated in the heating process is rolled to form a steel pipe of a predetermined shape. For the rolling, hot rolling including piercing rolling using a conventional Mannesmann plug mill or Mannesmann mandrel mill can be used. The predetermined shape refers to, for example, a cylindrical shape such as a steel pipe, including a steel pipe whose end diameter is smaller than the central diameter and a cylinder shape such as a pressure vessel. For example, the cylindrical steel pipe preferably has an outer diameter of 200 to 600 mm and an axial length of 500 to 12,000 mm. Furthermore, for a steel pipe whose end diameter is smaller than the central diameter, the central outer diameter is preferably 200 to 600 mm, the end diameter is 50 to 550 mm, and the axial length is preferably 500 to 12,000 mm. The cylinder preferably has an outer diameter of 200 to 600 mm and a length in the axial direction of 500 to 12,000 mm. The hot rolling also includes a method in which a step of rolling a billet into a steel pipe shape (hot working step) and a pipe expansion step are carried out simultaneously. If necessary, a sizing step for adjusting the plate thickness may be carried out after the reheating step described below.

[0058] After hot rolling, the resulting seamless steel pipe is subjected to a cooling treatment in which the pipe is cooled at a cooling rate equal to or greater than air cooling until the surface temperature reaches 200°C or lower.

[0059] Cooling treatment after hot rolling: average cooling rate: air cooling or more, cooling stop temperature: 200°C or less. Within the range of the component composition of the present invention, if cooling is performed at an average cooling rate equal to or greater than air cooling after hot rolling, a structure having martensite as the main phase can be obtained. If air cooling (cooling) is stopped when the surface temperature exceeds 200°C, the transformation may not be completely completed. Therefore, the cooling treatment after hot rolling is performed at an average cooling rate equal to or greater than air cooling until the surface temperature reaches 200°C or less. In addition, in the present invention, a "cooling rate equal to or greater than air cooling" refers to 0.1°C / s or more. If the average cooling rate is less than 0.1°C / s, the metal structure after cooling becomes non-uniform, and the metal structure after subsequent heat treatment becomes non-uniform. The average cooling rate is preferably 1.0°C / s or more, more preferably 10.0°C / s or more. Although there is no particular upper limit, the average cooling rate is preferably 1000.0°C / s or less. The above-mentioned average cooling rate refers to the average cooling rate of Ac ３ This is the average value of the cooling rate from the transformation point to 200°C.

[0060] [Heat treatment process] Reheating temperature for quenching treatment: Ac ３ Transformation point or higher and 1000°C or lower When performing quenching, the reheating temperature is Ac ３ Below the transformation point, the steel is not heated to the austenite single phase region, and therefore a structure with martensite as the main phase cannot be obtained. ３ The reheating temperature is preferably Ac ３ +30°C or higher, more preferably Ac ３ +50°C or higher. However, Ac ３ Point +30℃, Ac ３ For component systems where the temperature +50°C is higher than 1000°C, ３ Point +30℃ or higher, Ac ３A temperature higher than the hardening point +50°C is not applicable. On the other hand, if the temperature exceeds 1000°C, the crystal grains become coarse and the toughness decreases, and in addition, the oxide scale on the surface becomes thick and easily peels off, which causes defects on the steel sheet surface. Furthermore, the load on the heat treatment furnace becomes excessive, which is also a problem from the viewpoint of energy conservation. For these reasons, and from the viewpoint of energy conservation, the reheating temperature for quenching treatment is limited to 1000°C or less. Note that the temperature is preferably 980°C or less, and more preferably 950°C or less.

[0061] After reheating, the steel is subjected to quenching treatment. The quenching treatment is performed by rapidly cooling the steel until the surface temperature reaches 200°C or less. ３ The average cooling rate is preferably 5.0°C / s or more, and more preferably 10.0°C / s or more, from the transformation point to 200°C. The average cooling rate is not particularly limited, but the average cooling rate is preferably 1000.0°C / s or less. In addition, after satisfying the above, the temperature at the center of the plate thickness is ３ It is preferable to water-cool the material at an average cooling rate of 2.0°C / s or more to a temperature between the transformation point and 400°C or less. There is no particular upper limit, but the average cooling rate is preferably 1000.0°C / s or less. It is also preferable to cool the material to a surface temperature of 100°C or less. It is preferable that the surface temperature after cooling is low, and it is preferable to cool the material to room temperature. The quenching treatment may be repeated two or more times.

[0062] In addition, Ac ３ The transformation point is calculated using the following formula:

[0063] A.C. ３ Transformation point (℃)=937-476.5C+56Si-19.7Mn-16.3Cu-4.9Cr-26.6Ni+38.1Mo+124.8V+136.3Ti+198Al+3315B (Here, C, Si, Mn, Cu, Cr, Ni, Mo, V, Ti, Al, B: content of each element (mass%)) Ac ３In calculating the transformation point, if an element described in the above formula is not contained, the content of that element is set to 0%.

[0064] After cooling at a rate faster than air cooling, the steel is tempered at a temperature in the range of 600 to 740°C.

[0065] Tempering temperature: 600-740°C. Tempering is performed to reduce dislocation density and precipitate Mo precipitates, thereby improving toughness and hydrogen embrittlement resistance. At tempering temperatures below 600°C, the reduction in dislocations and the precipitation of Mo precipitates are insufficient, making it impossible to ensure excellent hydrogen embrittlement resistance. Therefore, the tempering temperature is set to 600°C or higher. The tempering temperature is preferably set to 620°C or higher, more preferably 640°C or higher, and even more preferably 660°C or higher. On the other hand, temperatures above 740°C result in significant softening of the structure, making it impossible to ensure the desired high strength. For this reason, the tempering temperature is limited to 740°C or lower. The tempering temperature is preferably set to 710°C or lower. The tempering temperature is more preferably set to 700°C or lower, and even more preferably to 680°C or lower.

[0066] The average heating rate to reach the tempering temperature is 0.5°C / min or more. Mo precipitates precipitate and increase in size during the tempering heating process. Therefore, if the heating rate to reach the specified temperature in the tempering treatment is slow, the precipitates will become too large, and the desired hydrogen embrittlement resistance will not be obtained. Therefore, the average heating rate to reach the tempering temperature is set to 0.5°C / min or more, preferably 1.0°C / min or more, and more preferably 2.0°C / min or more. Most preferably, it is set to 5.0°C / min or more. Although there is no particular upper limit, if the heating rate is too fast, uneven temperature distribution will occur, resulting in inhomogeneity in the material structure, so 50.0°C / min or less is preferred.

[0067] Holding time at tempering temperature: 10 minutes or more and less than 60 minutes Mo precipitates are most likely to precipitate during tempering. If this time is too short, they will not precipitate sufficiently and the desired hydrogen embrittlement resistance will not be obtained. The holding time at tempering temperature is 10 minutes or more. The holding time at tempering temperature is preferably 15 minutes or more, and more preferably 20 minutes or more. Furthermore, if the holding time at tempering temperature is too long, the size of the precipitates will become too large, so it is set to less than 60 minutes. Note that the holding time increases energy costs, so the tempering time is preferably less than 50 minutes, more preferably less than 40 minutes, and even more preferably less than 30 minutes.

[0068] In order to stably ensure the desired properties, after hot rolling, the steel is subjected to a cooling treatment in which it is cooled at a rate faster than air cooling, and then reheated and quenched once or more times by water cooling or the like, and then the above-mentioned tempering treatment is performed. There is no particular upper limit to the number of quenching treatments, but it is preferable to perform the quenching treatment five times or less.

[0069] After the quenching and tempering treatments, the steel pipe may be subjected to a warm or cold straightening treatment to correct any shape defects, if necessary.

[0070] Examples verifying the effects of the present invention will be described below. The following description shows a preferred example of the present invention, and the present invention is not limited to these examples. In the following examples, a manufacturing method and characteristic evaluation of seamless steel pipes for actual steel structures were investigated. Table 1 shows the chemical compositions of steels No. 1 to 24. Table 2 shows the tempering conditions, area fraction of tempered martensite, prior austenite grain size number, the proportion by mass of Mo contained in precipitates out of Mo contained in the steel, the proportion of Mo contained in precipitates with a diameter of 50 nm or less out of Mo contained in precipitates, TS, and relative reduction of area (RRA), for each of Nos. 1 to 24.

[0071]

[0072]

[0073] Billets having the chemical compositions shown in Nos. 1 to 24 in Table 1 were produced at a casting speed of 0.6 m / min, and the billets were heated to 1250°C, hot worked, and expanded to obtain seamless steel pipes. The seamless steel pipes were produced under conditions in which expansion was completed at 820°C or higher, and after hot working, cooling was carried out at a cooling rate faster than air cooling to a temperature at which the surface temperature was 200°C or lower. The obtained steel pipes were subjected to Ac ３ For steel pipes with a transformation point of 950°C or less, heating is maintained at 950°C. ３ Steel pipes with a transformation point exceeding 950°C were heated and held at 1000°C, then water-cooled to 200°C or less at a rate of 5.0°C / s, and then tempered. The tempering process was carried out at the heating rate, holding temperature, and holding time shown in Table 2. The tempering temperature was adjusted so that the tensile strength was in the range of 850 to 950 MPa. The metallographic structure and mechanical properties of the obtained steel pipes were evaluated.

[0074] Furthermore, steel pipes No. 25 to 39 in Table 3 were obtained by producing any of billets No. 5, 8, and 12 having the chemical compositions shown in Table 1 at various casting speeds, heating the billets to 1250°C, and expanding them to obtain seamless steel pipes. The steel pipes were produced under conditions in which expansion was completed at 820°C or higher, and after hot working, cooling was carried out at a cooling rate faster than air cooling to a temperature at which the surface temperature was 200°C or lower. The obtained steel pipes were subjected to Ac ３ For steel pipes with a transformation point of 950°C or less, heating is maintained at 950°C. ３ Steel pipes with transformation points exceeding 950°C were heated and held at 1000°C, then water-cooled to 200°C or less at a rate of 5.0°C / s, and then tempered under the conditions shown in Table 3. The metallographic structure and mechanical properties of the obtained steel pipes were evaluated.

[0075] The evaluation method is as follows.

[0076] Measurement of Martensite Area Fraction The metallographic structure at 1 / 4 of the wall thickness on the inner surface of the obtained steel pipe was evaluated as follows. Samples were taken from cross sections parallel to the longitudinal and thickness directions of the steel pipe so that the observation surfaces were the 1 / 4 of the wall thickness on the inner surface and the center of the wall thickness. The cross sections of the taken samples were etched using a 3 vol% nital solution. Scanning electron microscope photographs were taken at an appropriate magnification between 1000 and 5000 times, and tempered martensite, ferrite, bainite, and pearlite were observed. Tempered martensite was visually determined by comparing with the microstructure photographs in Reference 2. The microstructure fraction was determined by image analysis using images obtained by dividing the SEM photographs into regions based on the above judgment. The martensite and other regions were binarized to determine the tempered martensite fraction, which was then used as the area fraction of tempered martensite. [Reference 2] Japan Heat Treatment Association (author), Introduction to the Structure and Properties of Metallic Materials - Heat Treatment and Structure Control to Make the Most of Materials, 2004

[0077] Evaluation of Prior Austenite Grain Size Prior austenite (γ) was measured by polishing a cross section (C cross section) of a test piece for microstructural observation that was perpendicular to the longitudinal direction of the pipe, etching it with picral (picral (a mixed liquid of picric acid and ethanol)) to reveal the prior γ grain boundaries, and observing it using an optical microscope (magnification: 1000x), and photographing it in three or more locations. For the obtained microstructural photographs, the grain size number of the prior γ grains was determined using the intercept method in accordance with the provisions of JIS G 0551. The average value determined above was taken as the grain size number of the prior γ grains of each steel pipe.

[0078] Measurement method for Mo precipitates in steel material The measurement method for Mo precipitates in steel material collected from steel pipes is as follows. Mo precipitates were identified by an extraction method in which the steel material was electrolyzed and the resulting precipitates were filtered. A 10 mm square sample collected from a cross section perpendicular to the rolling direction of the steel pipe (a cross section perpendicular to the tube axis direction: C cross section) was dissolved by constant-current electrolysis using a 10% AA-based electrolyte, and the steel billet was then placed in a 0.05 wt % aqueous solution of sodium hexametaphosphate and irradiated with ultrasound to extract the precipitates. The solution was filtered through a filter with a filter diameter of 50 nm to obtain precipitates of 50 nm or less. Precipitates of 50 nm or less that passed through the filter and precipitates of larger than 50 nm on the filter were subjected to white smoke treatment using sulfuric acid, perchloric acid, and nitric acid, followed by dissolution in hydrochloric acid. The precipitate solution and the electrolyte containing the dissolved components were then subjected to concentration analysis by ICP to calculate the Mo concentration (mass%) and solute Mo concentration (mass%) in the precipitates of each size. The total Mo amount in the steel was calculated by adding up the Mo amounts and solute Mo amounts contained in all the precipitates calculated as above, and the Mo amount contained in all precipitates / total Mo amount and the Mo amount contained in precipitates of 50 nm or less / all precipitates were calculated.

[0079] Mechanical Property Evaluation Test specimens were taken from a cross section perpendicular to the steel pipe axis (C direction) with the center at 1 / 4 of the wall thickness from the inner surface of the steel pipe, with the longitudinal direction of the test specimen aligned in the C direction. Bar-shaped test specimens as specified in JIS Z 2201 "Tensile Test Specimens for Metallic Materials" were used. The test was performed using the method specified in JIS Z 2241, with the maximum load being the TS of the steel pipe. While it is preferable to center the test specimen at the 1 / 4 of the wall thickness, for steel pipes with a small wall thickness (e.g., a wall thickness of 45 mm or less), a method of not centering the test specimen at the 1 / 4 of the wall thickness position can also be used.

[0080] The hydrogen embrittlement resistance was evaluated from the relative reduction of area (RRA) of the test piece after a slow strain rate tensile test in hydrogen gas in accordance with ASTM G 142. In the atmosphere, the steel material undergoes plastic deformation, and the area of ​​the fracture surface becomes smaller. ａｉｒOn the other hand, since the elongation of steel material decreases in hydrogen, the material breaks before it is reduced in size, and the area of ​​the fractured surface remains large. Therefore, the reduction of area φ of the fractured surface after the test in hydrogen Ｈ The relative reduction of area (RRA) is calculated as follows: RRA = φ Ｈ / φ ａｉｒ The relative reduction of area obtained from a slow strain rate tensile test (tensile speed 0.002 mm / s) at room temperature under 105 MPa hydrogen gas is shown in Table 2. The larger the RRA, the better the hydrogen embrittlement resistance, and in this evaluation, a value of 60% or more was considered good. ａｉｒ is the cross-sectional area of ​​the test specimen after the test in air / the cross-sectional area before the test, and φ Ｈ is the cross-sectional area of ​​the specimen after the test in hydrogen / the cross-sectional area before the test.

[0081] All of the inventive examples satisfied the conditions of a TS of 850 MPa or more in a tensile test in air and an RRA of 60% or more in a slow strain rate tensile test in hydrogen gas.

[0082]

Claims

1. In mass percent, C: 0.20-0.50%, Si: 0.05-2.00%, Mn: 0.30 to 1.50%, P: 0.015% or less, S: 0.005% or less, Al: 0.005-0.150%, N: 0.006% or less, Cr: more than 0.2% but not more than 1.7%, Mo: more than 1.0% and less than 3.0%, Nb: 0.001-0.020%, B: 0.0003 to 0.0030%, O: 0.0030% or less, Ti: 0.003 to 0.025%; The ratio of the Mo content to the C content, Mo / C, is in the range of more than 2.0 to 12.0, The balance is Fe and unavoidable impurities, The steel has a structure in which the area ratio of tempered martensite is 95% or more, Of the Mo contained in the steel, 50% or more by mass is contained in precipitates, Furthermore, of the Mo contained in the precipitates, 50% by mass or more is contained in precipitates having a diameter of 50 nm or less, A high-strength seamless steel pipe for high-pressure hydrogen containers having a tensile strength TS of 850 MPa or more.

2. In addition to the above composition, further, in mass%, V: 0.30% or less, Cu: 1.00% or less, Ni: 2.0% or less, W: 3.0% or less 2. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, which contains one or more selected from the following:

3. In addition to the above composition, further, in mass%, H: 0.0010% or less 2. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, which contains

4. In addition to the above composition, further comprising, in mass %, H: 0.0010% or less 3. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 2, which contains

5. In addition to the above composition, further, in mass%, Ca: 0.0005-0.005% 5. The high-strength seamless steel pipe for high-pressure hydrogen containers according to claim 1, further comprising:

6. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers according to any one of claims 1 to 4, A steel pipe material having the above composition is cast into a slab, and the slab is heated at a temperature in the range of 1050 to 1350°C; The cast piece is hot-rolled to form a seamless steel pipe of a predetermined shape. After the hot rolling, the seamless steel pipe is cooled at an average cooling rate equal to or faster than air cooling until the surface temperature is 200° C. or less; After cooling, Ac ３ Reheating to a temperature in the range of from the transformation point to 1000°C, The steel is subjected to a quenching treatment at least once, in which the steel is rapidly cooled to a surface temperature of 200°C or less. After the quenching treatment, a tempering treatment is performed by heating to a tempering temperature of 600 to 740°C. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers, wherein the average heating rate until the tempering temperature is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

7. A method for producing a high-strength seamless steel pipe for a high-pressure hydrogen container according to claim 5, comprising the steps of: A steel pipe material having the above composition is cast into a slab, and the slab is heated at a temperature in the range of 1050 to 1350°C; The cast piece is hot-rolled to form a seamless steel pipe of a predetermined shape. After the hot rolling, the seamless steel pipe is cooled at an average cooling rate equal to or faster than air cooling until the surface temperature is 200° C. or less; After the cooling, the mixture is reheated to a temperature in the range of from the Ac3 transformation point to 1000°C, The steel is subjected to a quenching treatment at least once, in which the steel is rapidly cooled to a surface temperature of 200°C or less. After the quenching treatment, a tempering treatment is performed by heating to a tempering temperature of 600 to 740°C. A method for producing a high-strength seamless steel pipe for high-pressure hydrogen containers, wherein the average heating rate until the tempering temperature is reached is 0.5°C / min or more, and the holding time at the tempering temperature is 10 minutes or more and less than 60 minutes.

8. 7. The method for producing a high-strength seamless steel pipe for a high-pressure hydrogen container according to claim 6, wherein the casting is performed at a casting speed of 1.8 m / min or less.

9. A method for manufacturing a high-strength seamless steel pipe for high-pressure hydrogen containers as described in claim 7, wherein the casting speed is 1.8 m / min or less during the casting.